This invention relates to anode monitoring systems and anode monitoring methods for monitoring the integrity of anodes provided on a metallic structure for cathodic protection purposes. Examples of such structures are pipelines and components used with pipeline systems such as trees, manifolds and processing plants. This invention also relates to subsea pipeline power transmission systems, methods and apparatus.
The term subsea is used in this application as this is conventional terminology, however, it will be understood that this covers any underwater situation.
A subsea pipeline is typically protected by the use of cathodic protection. This means that sacrificial anodes are disposed at spaced locations along its length. The continued presence and effectiveness of the anodes is essential to the functioning of the cathodic protection. Thus, to ensure the continued integrity of the pipe itself, the anodes must be regularly inspected. At present this is either done by the use of remotely operated vehicles and/or potential surveys. Each of these methods is extremely costly and can only be performed when weather conditions allow.
In many circumstances where subsea pipeline systems are used, there is a desire to operate equipment at locations which, in the general sense, are remote. That is to say, although the equipment is situated adjacent to the pipeline itself it is not near any other facility or infrastructure. Such pieces of equipment might, for example, be sensors which monitor the integrity or operation of the pipeline system.
One of the problems with such remote pieces of equipment is providing a suitable power source. Whilst batteries can be used these are unattractive for various reasons including their limited life, their expense and environmental concerns.
It is an object of this invention to provide an anode integrity monitoring technique which alleviates at least some of the problems of the existing techniques.
It will be appreciated that the anodes may become non-effective in a number of ways, for example the anode may become totally detached from the pipeline, it may lose effective electrical contact with the pipeline or may have disintegrated to such an extent that it ceases to be effective. It is desirable to be able to detect when any of these events has occurred.
It is another object of the present invention to provide methods, systems and apparatus which allow the supply of power to remote equipment in subsea pipeline systems.
According to a first aspect of the present invention there is provided an anode monitoring system for monitoring the integrity of anodes provided on a metallic structure for cathodic protection purposes comprising a signal circuit having at least one signal path comprising the metallic structure and a selected anode whereby the characteristics of the signal circuit depend on the effectiveness of the selected anode, signal generation means for generating and applying a signal to the signal circuit, and a central station for monitoring signals on the signal circuit to thereby determine whether the selected anode is effective.
According to a second aspect of the present invention there is provided an anode monitoring method for monitoring the integrity of anodes provided on a metallic structure for cathodic protection purposes comprising the steps of:
generating a signal and applying said signal to a signal circuit, the signal circuit comprising at least one signal path comprising the metallic structure and a selected anode whereby the characteristics of the signal circuit depend on the effectiveness of the selected anode; and
monitoring signals on the signal circuit at a central station to thereby determine whether the selected anode is effective.
Preferably the signal generating means is arranged, when the selective anode is effective, to apply a signal to the signal circuit which is indicative of the effectiveness of the selected anode.
Preferably the signal generating means, or at least one component thereof is disposed at the selected anode.
The absence or defectiveness of the selected anode may be detectable as a break in the signal circuit. The break in the circuit may be detectable as the result of an inability to apply a signal to the signal circuit and/or an inability to receive a signal from the circuit. The absence or defectiveness of the selected anode may be detectable due to the absence of an expected signal. The expected signal may be that resulting from a change in the effective impedance of the signal circuit.
The signal circuit may comprise a return path via earth. Preferably the selected anode, when effective, provides a conduction path from the metallic structure to earth. Where the selected anode provides a path to earth, the absence or defectiveness of the selected anode may be detectable as the loss of an earth connection.
The signal circuit may comprise impedance means. The impedance means may be disposed between the selected anode and the remainder of the metallic structure. The impedance means may be provided in series between the selected anode and the metallic structure.
The impedance means may comprise isolation means.
The impedance means may comprise inductance means. The impedance means may comprise filter means. The impedance means may be arranged to give a high impedance to time varying signals within one or more selected ranges of frequencies and a low impedance to signals outside the selected range or ranges. The impedance means can be arranged so that the real part of the impedance is substantially zero. This means that there is little or no attenuation of the dc components of signals passing through the impedance means.
The use of inductance means and/or filter means has advantages when the metallic structure is used to carry signals because these means can be chosen to offer high impedance to the time varying signals used for signalling thereby reducing losses, whilst offering low impedance to the currents used for cathodic protection.
Transmitting means and receiving means may be provided for allowing data to be transmitted along the metallic structure. The transmitting and receiving means may be provided to assist the anode monitoring operation and/or to provide a distinct data transmission function.
The transmitting means and/or receiving means may be connected across the impedance means and arranged to transmit and/or receive signals across the impedance means.
Where signals are received across the impedance means, the use of filter means as the impedance means has an additional advantage that noise generated outside the frequency band of interest will be attenuated before it enters the receiver.
In some embodiments the signal generating means comprises transmitting means, the signal circuit comprises an earth return path so that the transmitting means requires an earth connection and the selected anode is arranged, when effective, to provide the earth connection so allowing transmission of a signal indicative of the anode""s effectiveness which is detectable at the central station. When the selected anode is defective or absent the transmitting means has no earth reference so that no signal is transmittable by the transmitting means. Therefore if the signal is absent it can be determined that the selected anode is defective or absent. In such embodiments the transmitting means is preferably connected across the impedance means.
In other embodiments the signal generating means comprises reference signal generating means arranged to apply a reference signal to the signal circuit and effective impedance varying means for varying the effective impedance of the signal circuit in accordance with data to be transmitted, the central station comprises monitoring means for monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit and the signal circuit is arranged such that defectiveness or absence of the selected anode causes a break in the signal circuit whereby non-effectiveness of the selected anode is detectable at the central station due to the absence of changes in the reference signal.
In such embodiments the reference signal generating means may be arranged to be locatable at a position which is remote from the selected anode. The impedance varying means may be located adjacent the selected anode.
Preferably the signal circuit comprises a plurality of signal paths each comprising the metallic structure and a respective anode. The subsidiary features defined above in relation to the selected anode apply equally to each of the respective anodes in a system with a plurality of signal paths. Independent signal generating means or at least one independent component of the signal generating means may be disposed at each anode.
Different data and/or a different signal and/or a different frequency may be associated with each of the respective anodes. The system may be arranged so that signals associated with each anode are generated at different times. The signals may be randomly generated. In this way, for example, when a particular anode is non-effective and hence its associated data/signal is not received at the central station it is possible to determine which anode it is which is non-effective.
According to a third aspect of the present invention there is provided a data transmission system comprising transmitting means, receiving means and a metallic structure which is primarily intended for another purpose but which in use acts as a signal channel between the transmitting means and the receiving means, wherein the metallic structure includes at least one anode provided for cathodic protection purposes and impedance means are disposed between the metallic structure and the anode.
The data transmission system may comprise a signal circuit comprising the metallic structure and a return path. The return path may be via earth. The signal circuit may comprise the anode. Preferably the anode provides a path from the metallic structure to earth.
The impedance means may be provided in series between the respective anode and the metallic structure.
The impedance means may comprise inductance means. The impedance means may comprise filter means. The impedance means may be arranged to have a high impedance to time varying signals within one or more selected ranges of frequencies and a low impedance to signals outside the selected range or ranges. The use of the inductance means or filter means gives the advantages discussed above.
According to a fourth aspect of the present invention there is provided apparatus for use with a metallic structure in carrying out the first, second or third aspects of the invention.
In all of the above aspects of the invention the metallic structure may comprise a pipeline, for example, a subsea pipeline of the type used for conveying oil or gas. The metallic structure may comprise a processing plant and/or a tree and/or a manifold.
According to a fifth aspect of the present invention there is provided a subsea pipeline power transmission system comprising a pipeline, an electrical power supply connected to the pipeline at a first location, and connection means provided on the pipeline at a second location for connection of a load to the pipeline to allow the load to receive electrical power from the power supply via the pipeline wherein the pipeline has a plurality of cathodic protection anodes, each of which is electrically connected via respective impedance means to the pipeline.
According to a sixth aspect of the present invention there is provided a method of subsea pipeline power transmission along a pipeline having a plurality of cathodic protection anodes comprising the steps of:
applying electrical power to the pipeline at a first location; and
electrically connecting a load to be supplied to the pipeline at a second location;
wherein each anode is electrically connected via respective impedance means to the pipeline.
According to a seventh aspect of the present invention there is provided apparatus for use in a subsea pipeline power transmission system or method comprising:
an anode arrangement comprising, a sacrificial anode arranged for mounting on a pipeline and impedance means having one terminal connected to the anode and another terminal arranged for connection to said pipeline; and
an electrical power supply arranged for electrical connection to a pipeline.
According to an eighth aspect of the present invention there is provided an anode arrangement for use in a subsea pipeline power transmission system, the arrangement comprising, a sacrificial anode arranged for mounting on a pipeline and impedance means having one terminal connected to the anode and another terminal arranged for connection to said pipeline.
The anode arrangement may include further terminals allowing the connection of a load across the impedance means.
The impedance means may comprise inductance means. Preferably the impedance means comprises filter means. The impedance means, especially when comprising filter means, may be arranged to give a high impedance to time varying signals within one or more selected ranges of frequencies and a low impedance to signals outside the selected range or ranges. The impedance means can be arranged so that the real part of the impedance is substantially zero. This means that there is little or no attenuation of the dc components of signals passing through the impedance means.
The use of inductance means and particularly filter means has advantages when the metallic structure is used to carry power currents because these means can be chosen to offer high impedance to the time varying signals used for power supply thereby reducing losses, whilst offering low impedance to the currents used for cathodic protection. Minimising losses is particularly important when transmitting power rather than merely trying to detect a signal. Limiting loss to a realistic level is necessary to give a practical system.